Trouble With Correlated Subquery, Example Using Northwind

Suppose I have this query, which shows each order and the price of the most expensive item in each order:

SELECT Ord.OrderID, Ord.OrderDate,     MAX(OrdDet.UnitPrice) AS maxUnitPrice
FROM Northwind.dbo.[Order Details] AS OrdDet     INNER JOIN     Northwind.dbo.Orders AS Ord    ON Ord.OrderID = OrdDet.OrderID
GROUP BY Ord.OrderID, Ord.OrderDate

I need to also show the ProductID that has MaxUnitPrice from the Order
Details.  I can't just add ProductID to the select list because
I'd have to group by it, and then I'd get a row for each product,
instead of a row for each order... I think I need a correlated subquery
but can't work out how to do it!